Theoretical Studies of Polar Systems near Ferroelectric Quantum Critical Points

Premala Chandra, Rutgers, The State University of New Jersey, New Brunswick (Principal Investigator)

The connection between ferroelectricity and quantum criticality is not obvious.  Interest in insulating ferroelectrics is often motivated by room-temperature functionalities, whereas a crucial stimulus for research in quantum criticality is its link with unconventional superconductivity.  At a regular phase transition, thermal fluctuations melt the long-range ordering.  By contrast quantum criticality, associated with continuous quantum phase transitions, occurs at zero temperature where thermal fluctuations are absent; here the phase change is driven by zero-point fluctuations whose magnitude can be tuned by pressure or field.

Experimentally most ferroelectric systems display first-order classical phase transitions due to strong electromechanical coupling, and yet in many cases low-temperature experiment suggest pressure-induced quantum criticality associated with zero-temperature continuous transitions.  A key focus of this research will be to develop a theoretical framework for this phenomenon, initially with a simplified model that is then “decorated” with more realistic features.  Next physical settings for this behavior will be identified, and signature experimental probes will be developed to identify ferroelectric quantum criticality in conjunction with systematic laboratory measurements.  In general terms, another class of materials is important for the study, both theoretical and experimental, of universality in quantum critical systems.

Recently systematic deuteration of a family of paraelectric salts suggests that they can be tuned to their ferroelectric quantum critical points.  In a close theoretical-experimental collaboration, the possibility of using these materials for compact, cryogenic solid-state refrigeration will be explored. Because electrocaloric cooling and pyroelectricity are closely related, these deuterated salts may also be good candidates for converting heat to electricity; such pyroelectric harvesting could be used in satellite infrared detectors where their radiation tolerance is an added advantage.

The doping of polar materials near ferroelectric quantum critical points leads to quantum critical polar metals where structural transitions replace formerly ferroelectric ones; these quantum critical polar metals are the final topic in this research project.  The effect of charge carriers on these quantum structural transitions and the nature of the electron-electron interactions in this novel polarized quantum critical fluid will be studied. Detailed comparison to specific experiments using materials-specific parameters from ab initio calculations will be performed whenever possible with the aim of exploring possible emergent exotic quantum phases.